The ''Labarum'' was originally a Roman military ensign, which is described to have been a more distinguished form of the [[w:Vexillum|Vexillum]], or cavalry standard.<ref group="note">That the ''Labarum'' dated its designation as the imperial standard from an early period of the empire, is a supposition confirmed by a colonial medal of [[w:Tiberius|Tiberius]] (dedicated to that Prince by ''Caesarea-Augusta'' (Saragozza)), on which may be discerned the form of that ensign. The Labarum is also to be found in the left hand of emperors; on some military figures; and on coins of Nero, Domitian, Trajan, Hadrian, Antoninus Pius, Marcus Aurelius, Commodus, Septimius Severus, and other princes anterior to Constantine. In addition, a vexillum or cavalry standard, resembling the Labarum, appears on several colonial coins, such as those from Acci, Antiochia Pisidiae, and Caesarea-Augusta. (<ref>''[http://www.forumancientcoins.com/numiswiki/view.asp?key=labarum Labarum].'' '''Numiswiki: The Collaborative Numismatics Project.)'''</ref> Like the other standards, it was an object of religious veneration amongst the soldiers, who paid it divine honours. As an imperial standard, the labarum was only hoisted when the Emperor was with the army.

Today, the term "labarum" is generally used for any ecclesiastical banner, such as those carried in religious processions.<ref group="note">Some Protestant Christians (especially Restorationists) reject the use of Labarum Christogram (with the Chi-Rho monogram) due to its supposed pagan origins and lack of use by the earliest Christians. Supporters point out that use of the Labarum was already in widespread use by Christians by the mid-fourth century, mostly on sarcophagi.</ref>